A simple frequency analysis shows the true rotational speed of these two
drives. The pitch of the sound corresponds to the frequency of the vibration, which is caused by its rotational speed. The former, Hz, is cycles per second, while the latter is rotations per minute.

Caviar
Green 2TB Frequency / Amplitude graph.

Our Caviar Green sample had a sharp spike at 90 Hz (cycles per second). Multiplying
that number by 60 (to get cycles per minute) yielded a rotation speed
of 5400 RPM. Despite Western Digital's claims about IntelliPower, we have yet
to hear a single case of a Green Power drive running at a speed other than 5400
or 7200 RPM.

Pipeline HD 500GB Frequency / Amplitude graph.

Our 500GB Pipeline sample generated a a spike at 96 Hz, which means its disks
spin at around 5800 RPM.